While looking at the USGS Earthquake(EQ) Hazard map today, I noticed 235 EQ's have occurred in?

You've asked a totally reasonable question and Nature Nate is correct that seismographs can pick up explosions and the like. But seismologists have asked themselves the same question and developed methods for distinguishing large explosions from small earthquakes.If you click on the earthquake localities on map at the USGS url that you give--you will notice that you get information on the depth of the starting point of the earthquake. Click around and you'll see that the huge majority (all?) are several kilometers (miles) below the surface--so they're natural, not manmade.